Secondary battery

ABSTRACT

The present invention relates to a secondary battery in which a stacked electrode assembly having a cathode, an anode and a separator is accommodated together with an electrolytic solution between exterior members. In the present invention, the secondary battery has a plurality of joint parts at which the outer peripheral portion of the separator is joined with the exterior members and a holding part formed at least between the joint parts so as to hold therein the electrolytic solution, wherein a sum of perimeters of the joint parts is longer than a perimeter of a rectangle of minimum area enclosing therein all of the joint parts. In this configuration, it is possible to refill the stacked electrode assembly with the electrolytic solution and protect the joint parts from breakage while preventing displacement of the stacked electrode assembly in the secondary battery.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 13/885,572 filed May 15, 2013, which is the National StageApplication of International Application No. PCT/JP2011/072985, filedOct. 5, 2011, which claims priority to Japanese Patent Application Nos.2010-258365 filed Nov. 18, 2010 and 2011-185051 filed Aug. 26, 2011, theentire contents of all are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a secondary battery.

BACKGROUND ART

Provided is a high-output and/or high-capacity battery module in which aplurality of flat batteries having electrodes led out of exteriormembers are stacked and electrically connected in series and/or parallelto each other.

One example of flat battery usable in the battery module is anon-aqueous electrolyte secondary battery. The non-aqueous electrolytesecondary battery is a battery in which a stacked electrode assembly isaccommodated together with a non-aqueous electrolytic solution betweenexterior members. The stacked electrode assembly has a positiveelectrode, a negative electrode and a separator for preventing a shortcircuit between the positive and negative electrodes. For example,aluminum laminate sheets are usable as the exterior members. Outerperipheral portions of the aluminum laminate sheets are sealed by fusionbonding to form a battery package such that the stacked electrodeassembly can be accommodated in the battery package.

There is known a technique for fusion bonding an outer peripheralportion of the separator with the seal portions of the aluminum laminatesheets in order to prevent a displacement of the stacked electrodeassembly in the secondary battery (see Patent Document 1).

PRIOR ART DOCUMENTS Patent Document

Patent Document 1: Japanese Laid-Open Patent Publication No. H11-250873

SUMMARY OF THE INVENTION

In order to maintain the performance of the battery, it is important tocontain a predetermined amount of electrolytic solution in the stackedelectrode assembly. It is thus preferable to refill the stackedelectrode assembly with the electrolytic solution in the case where theamount of the electrolytic solution in the stacked electrode assemblybecomes insufficient due to some reason.

In the non-aqueous electrolyte secondary battery of Patent Document 1 inwhich the outer peripheral portion of the separator is joined with theexterior members, however, any consideration is not given to therefilling of the stacked electrode assembly with the electrolyticsolution. This may result in a deterioration of battery performanceduring long-term use.

For the purpose of prevention of battery performance deterioration inthe secondary battery in which the outer peripheral portion of theseparator is joined with the exterior member such as aluminum laminatesheets, it is also desirable to take measures to prevent the outerperipheral portion of the separator from being broken from the jointpart. Any measures against such breakage are not however taken in thenon-aqueous electrolyte secondary battery of Patent Document 1. Thismakes it difficult to increase the operation life of the battery.

The present invention has been made to solve the above prior artproblems. It is accordingly an object of the present invention toprovide a secondary battery in which a separator of a stacked electrodeassembly is joined at an outer peripheral portion thereof betweenexterior members so as to allow refilling of the stacked electrodeassembly with an electrolytic solution, favorably prevent the separatorfrom being broken from its joint part and thereby maintain batteryperformance during long-term use.

In order to achieve the above object, there is provided according to thepresent invention a secondary battery, comprising: a stacked electrodeassembly having a positive electrode, a negative electrode and aseparator; an electrolytic solution; and exterior members accommodatingtherebetween the stacked electrode assembly together with theelectrolytic solution, wherein the secondary battery comprises: aplurality of joint parts at which an outer peripheral portion of theseparator is joined with the exterior members; and a holding part formedat least between the joint parts so as to hold therein the electrolyticsolution; and wherein a sum of perimeters of the joint parts is longerthan a perimeter of a rectangle of minimum area enclosing therein all ofthe joint parts.

In the present invention, the outer peripheral portion of the separatoris joined with the exterior members by forming a plurality of separatejoint parts, rather than by forming a continuous single joint part,while forming the holding part at least between the joint parts to holdtherein the electrolyte solution. In this configuration, the stackedelectrode assembly can be refilled with the electrolyte solution fromthe holding part between the joint parts. Further, the sum of theperimeters of the joint parts is made longer than the perimeter of therectangle of minimum area enclosing therein all of the joint parts sothat, even though the separator becomes smaller in thickness at thejoint parts, the occurrence of breakage in the joint parts can beprevented by improvement in strength against tensile force in thepresent invention. It is therefore possible to maintain the performanceof the secondary battery during long-term use.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a battery module according to oneexemplary embodiment of the present invention.

FIG. 2 is a perspective view of a cell unit in the battery module ofFIG. 1.

FIG. 3 is a perspective exploded view of the cell unit of FIG. 2 asviewed from the back side.

FIG. 4 is a perspective view of a flat battery according to oneexemplary embodiment of the present invention.

FIG. 5 is a plan view of substantial part of the flat battery, showingjoint parts at which the outer peripheral portion of a separator isjoined with exterior members, according to one embodiment of the presentinvention.

FIG. 6A is a section view taken along line 6A-6A of FIG. 5; and FIG. 6Bis a section view taken along line 6B-6B of FIG. 5.

FIG. 7A is an enlarged view of an area enclosed by broken line 7A inFIG. 5; and FIG. 7B is an enlarged view of an area enclosed by brokenline 7B in FIG. 5.

FIGS. 8A and 8B are schematic views showing how a tensile force acts onthe flat battery in a direction of outline arrow.

FIGS. 9A to 9E are schematic view showing examples of the shape andarrangement of a plurality of joint parts and the relationship of thesum (La) of the perimeters of the joint parts and the perimeter (Lb) ofa rectangle of minimum area encompassing all of the joint parts.

FIGS. 10A to 10C are schematic view showing examples of the shape andarrangement of a plurality of joint parts and the relationship of thesum (La) of the perimeters of the joint parts and the perimeter (Lb) ofa rectangle of minimum area encompassing all of the joint parts.

FIG. 11 is a plan view of substantial part of a flat battery, showingjoint parts, according to another exemplary embodiment of the presentinvention.

FIG. 12 is schematic views of joint parts formed in Examples andComparative Example.

DESCRIPTION OF EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will bedescribed below with reference to the drawings. It is noted that: in thedrawings, like parts and portions are designated by like referencenumerals to omit repeated explanations thereof; and the dimensions ofthe respective parts and portions may be exaggerated for purposes ofillustration and may be different from the actual dimensions. Further,x-axis and y-axis indicate the lateral and longitudinal of flat battery10, respectively, in the drawings.

As shown in FIGS. 1 and 2, battery module 100 includes cell unit 130formed with a plurality of flat batteries 10 (as secondary batteries),insulating cover 140 having electrical insulating properties and case120 accommodating therein cell unit 130 and insulating cover 140.Although battery module 100 can be used solely, it is feasible toprovide an assembled battery with desired current, voltage and capacitycharacteristics by series and/or parallel connection of a plurality ofbattery modules 100.

Case 120 has rectangular box-shaped lower case member 122 and lid-shapedupper case member 124. An edge portion of upper case member 124 is woundaround and fixed by crimping to an edge portion of a peripheral wall oflower case member 122. Each of lower case member 122 and upper casemember 124 is formed from a relatively thin steel plate or aluminumplate. Lower case member 122 and upper case member 124 have throughholes 126 formed in respective four corner portions thereof such thatstacked battery modules 100 can be maintained as the assembled batteryby insertion of bolts (not shown) in through holes 126. Herein,reference numerals 131 and 132 designate output terminals arranged toprotrude from front opening portions of lower case member 122.

As shown in FIG. 2, cell unit 130 has stacked body 132 in which aplurality of flat batteries 10 are electrically connected together and aplurality of spacers 160 and 161 supporting the batteries. Each ofspacers 160 and 161 has electrical insulating properties. Spacers 160are arranged on a front side of stacked body 132, whereas spacers 161(as a supporting member) are arranged on a back side of stacked body132.

For example, spacers 161 on the back side of stacked body 132 arepositioned in such a manner to hold outer peripheral portions 32 ofexterior members 30 of flat batteries 10 as shown in FIG. 3. Spacers 161have through holes 162 formed in longitudinally opposite end portionsthereof such that through holes 162 can be aligned with through holes126 on the back side of lower case member 122 and upper case member 124.

As shown in FIGS. 4 to 7, flat battery 10 is configured as e.g. alithium-ion secondary battery and has a structure that stacked electrodeassembly 20 is accommodated together with an electrolytic solutionbetween exterior members 30. Flat battery 10 includes electrodes 41 and42 (referred to as “tabs”) let to the outside from exterior members 30.In FIG. 5, reference numeral 21 designates a positive electrode or anegative electrode. For purposes of clarity, only separators 22 areillustrated in FIG. 6.

Stacked electrode assembly 20 includes a positive electrode orelectrodes, a negative electrode or electrodes and a separator orseparators 22 stacked alternately together. The positive electrode has apositive electrode active material layer formed of e.g. alithium-transition metal composite oxide such as LiMn₂O₄, whereas thenegative electrode has a negative electrode active material layer formedof e.g. a carbon material and a lithium-transition metal compositeoxide. Separator 22 is formed of e.g. porous PE (polyethylene) havingpermeability to allow permeation of an electrolyte material.

In terms of weight reduction and thermal conduction, exterior members 30are formed from sheets of e.g. polymer-metal laminate film in which ametal (or alloy) such as aluminum, stainless steel, nickel or cupper iscoated with an insulating material such as propylene film. Each ofexterior members 30 includes body portion 31 covering stacked electrodeassembly 20 and outer peripheral portion 32 extending along peripheraledge of body portion 31. Outer peripheral portions 32 are joined at partor the whole thereof by fusion bonding.

Tabs 41 and 42 are adapted to take an electric current out of stackedelectrode assembly 20 and arranged to extend on a front side of flatbattery 10.

Flat battery 10 has a plurality of joint parts 40 at which outerperipheral portions of separators 22 are joined with exterior members 30in order to prevent a displacement of stacked electrode assembly 20.Flat battery 10 further includes holding part 50 formed at least betweenjoint parts 40 to hold therein the electrolytic solution in such amanner that stacked electrode assembly 20 can be refilled with theelectrolytic solution. The joining can be done by thermal fusionbonding, ultrasonic fusion bonding, welding etc. In FIGS. 5, 7 and 8,joint parts 40 are indicated by hatching for ease of understanding.

More specifically, the outer peripheral portions of separators 22 arejoined with the exterior members 30 by forming a plurality of separatejoint parts 40 as shown in FIGS. 5, 6 and 7 rather than by forming acontinuous single joint part. The continuous single joint part ishereinafter sometimes simply referred to as “continuous joint part” forthe sake of simplicity.

The dimensions of joint parts 40 can be set, for example, as follows: W1(width)=2 mm, W2 (distance)=1 mm, W3 (length)=2 mm and W4 (distance fromthe fused joint (joint part 40) between the outer peripheral portions ofseparators 22 and exterior members 30 to the fused joint between outerperipheral portions 32 of exterior members 30)=5 mm as shown in FIGS.7(A) and (B). There is no particular limitation on the ratio of the sumof the areas of joint parts 40 to the total area of separator 22. Forexample, the ratio of the sum of the areas of joint parts 40 to thetotal area of separator 22 can be set to 0.1% to 1%.

In the case of a battery in which separators are joined between exteriormembers by a continuous joint part, the battery has a so-called bagstructure that an electrolytic solution is accumulated and sealed in aspace between the continuous joint part and the joint between outerperipheral portions of the exterior members. In such a structure, it isnot possible to make effective use of the sealed electrolytic solution.

On the other hand, separators 22 are joined with the exterior member 30by a plurality of joint parts 40 in the present embodiment. Asseparators 22 are not fused together in at least the region between thejoint parts 40, holding parts 50 are formed in a small space betweenseparators 22 in this region so as to hold therein electrolytic solution51 (see FIG. 6). Stacked electrode assembly 20 can be thus refilled withelectrolytic solution 51 from holding part 50 under the action ofcapillarity when the amount of electrolytic solution 51 in stackedelectrode assembly 20 becomes insufficient due to some reason. By suchrefilling of stacked electrode assembly 20 with electrolytic solution51, it is possible to maintain a state in which stacked electrodeassembly 20 contains a predetermined amount of electrolytic solution 51over a long period of time so that flat battery 10 can secure batteryperformance during long-term use.

Even when gas is accumulated between the stacked electrodes duringbattery manufacturing process, such accumulated gas can be easilyreleased from the power generating portion of the battery to outerperipheral portions 32 through region S1 between joint parts 40. Thismakes it possible to prevent a deterioration in power generatingefficiency caused by the accumulated gas.

As shown in FIGS. 5 and 7, not only region S1 between joint parts 40 butalso clearance region S2 from the uppermost one of the plurality ofjoint parts 40 in the drawing to exterior members 30, clearance regionS3 from the lowermost one of the plurality of joint parts 40 in thedrawing to the exterior members 30 and clearance region S4 between theplurality of joint parts 40 and the back-side ends of exterior members30 are adapted to serve as holding part 50. This makes it possible tohold a larger amount of electrolytic solution 51 for refilling ofstacked electrode assembly 20 so that flat battery 10 can secure batteryperformance during longer-term use.

As shown in FIGS. 3 to 5, outer peripheral portion 32 of exterior member30 includes a plurality of supporting parts 33 coupled by an adhesive tobattery-supporting spacers 161 and extension part 34 located betweensupporting parts 33 and extending radially outwardly of the battery. Theouter peripheral portion of separator 22 has tongue part 23 extendingtoward extension parts 34 of exterior members 30 such that joint parts40 are formed on tongue part 23. This makes it possible to makeeffective use of the dead space that is supported by spacers 161 anddoes not substantially contribute to power generation, increase the holdamount of electrolytic solution 51 and improve the operation lifetime ofthe battery.

Through holes 35 are formed in extension parts 34 such that pins inspacers 161 can be inserted in through holes 35 so as to restrict thesupporting position of flat battery 10 relative to spacers 161.

In FIG. 8(A), schematically indicated by a two-dot chain line isbreakage line 61 that could be developed at or around joint parts 40when a tensile force acts on separator 22 in the direction of outlinearrow 60. Under such a tensile force, a tensile stress is exerted oneach of joint parts 40 in the directions of solid-line arrows 62 asschematically indicated in FIG. 8(B).

When the tensile stress is excessively exerted on each joint part 40 inthe directions of solid-line arrows 62 in FIG. 8(B), there occursbreakage in separator 22 etc. along breakage line 61 in FIG. 8(A). Asbreakage line 61 extends along the periphery (three sides) of each ofjoint parts 40, the battery is made able to withstand a larger tensilestress by increasing the sum of the perimeters of joint parts 40.

It is thus preferable to increase the number of joint parts 40 and toincrease the sum of the perimeters of joint parts 40 as shown in FIG. 5in the case where a larger tensile stress acts on each joint part 40.

In particular, the sum (La) of the perimeters of joint parts 40 ispreferably made longer than the perimeter (Lb) of rectangle 63 ofminimum area enclosing therein all of joint parts 40. The configurationof rectangle 63 can be defined as indicated by a broken line in FIGS. 9and 10. By satisfaction of such a condition, separator 22 can attain animproved strength against tensile force during use of flat battery 10 ina vehicle such as automotive vehicle where vibrations occur duringrunning. This makes it possible to prevent the occurrence of breakage injoint parts 40 at which separator 22 is made thinner.

Herein, examples of the shape and arrangement of a plurality of jointparts 40 and the relationship of La and Lb are schematically shown inFIGS. 9(A) to (E) and FIGS. 10(A) to (C).

In FIG. 5 and FIGS. 9(A) and (C), joint parts 40 are all made the samein size and aligned in a line in the x-axis direction. The relationshipof La and Lb is either La<Lb (FIG. 9(C)) or La>Lb (FIG. 5 and FIG.9(A)). The present invention is not limited to this example.

It is feasible to align joint parts 40 of different sizes in a line inthe x-axis direction as shown in FIG. 9(B). The condition of La>Lb issatisfied in this example.

As shown in FIGS. 9(D) and (E), it is feasible to displace any one orones of joint parts 40 relative to the other joint parts 40 in they-axis direction rather than to arrange joint parts 40 in a line in thex-axis direction. The relationship of La and Lb is La>Lb (FIG. 9(D)) orLa<Lb (FIG. 9(E)).

Further, joint parts 40 may be formed along the x-axis direction andarranged in two columns in the y-axis direction as shown in FIG. 10(A).The condition of La>Lb is satisfied in this example. Even in such anarrangement, holding part 50 can be formed so as to hold thereinelectrolytic solution 51 and refill stacked electrode assembly 20 withelectrolytic solution 51 as long as there are some spaces left at leastbetween joint parts 40 in the vertical direction of the drawing.

As shown in FIG. 10(B), joint parts 40 may be arranged in two rows inthe x-axis direction and in two columns in the y-axis direction. In thisexample, the condition of La>Lb is satisfied.

As shown in FIG. 10(C), joint parts 40 may be arranged in two columns inthe y-axis direction with five joint parts 40 aligned in the x-axisdirection in the upper column. The condition of La>Lb is satisfied inthis example. The position of space between joint parts 40 in the uppercolumn are displaced in the x-axis direction from the position of spacebetween joint parts 40 in the lower column. Even in such an arrangement,holding part 50 can be formed so as to hold there in electrolyticsolution 51 and refill stacked electrode assembly 20 with electrolyticsolution 51.

The arrangement of joint parts 40 is not limited to the directionparallel to the x-axis or y-axis direction. Although not shown in thedrawings, it is feasible to arrange joint parts 40 in any directioninclined relative to the x-axis or y-axis direction.

As described above, flat battery 10 according to the present embodimenthas joint parts 40 at which the outer periphery of separator 22 isjoined with exterior members 30 and holding part 50 formed at leastbetween joint parts 40 so as to hold therein electrolytic solution 51and refill stacked electrode assembly 20 with electrolytic solution 51.It is therefore possible to maintain the state in which stackedelectrode assembly 20 contains a predetermined amount of electrolyticsolution 51 over a long period of time by refilling stacked electrodeassembly 20 with electrolytic solution 51 from holding part 50 betweenjoint parts 40, whereby the secondary battery can secure batteryperformance during long-term use. Further, it is easier to release thegas accumulated between the stacked electrodes to outer peripheralportions 32 through the region between joint parts 40 and is possible toprevent a deterioration in power generating efficiency caused by theaccumulated gas. It is furthermore possible to improve the strength ofseparator 22 against tensile strength and prevent the occurrence ofbreakage in joint parts 40 in which separator 22 is made smaller inthickness by controlling the sum (La) of the perimeters of joint parts40 to be longer than the perimeter of rectangle 63 of minimum areaenclosing therein all of joint parts 40.

As each of outer peripheral portions 32 of exterior members 30 has aplurality of supporting parts 33 coupled to battery-supporting spacers161 and extension part 34 located between supporting parts 33 andextending radially outwardly of the battery; and as the outer peripheralportion of separator 22 has tongue part 23 extending to betweenextension parts 34 of exterior members 30 such that joint parts 40 areformed on tongue part 23, it possible to make effective use of the deadspace that is supported by spacers 161 and does not substantiallycontribute to power generation, increase the hold amount of electrolyticsolution 51 and improve the operation lifetime of the battery.

Another Embodiment

Next, a flat battery according to another embodiment of the presentinvention will be explained below.

In general, flat battery 10 according to the present another embodimentincludes a plurality of joint parts 40 at which outer peripheralportions of separators 22 are each joined with exterior members 30 andholding part 50 formed between joint parts 40 as shown in FIG. 11. Jointparts 40 are arranged in joint regions 45, 46 and 47. In each of jointregions 45, 46 and 47, joint parts 40 are arranged adjacent to and atdistance W2 away from each other. One joint region 44 is located at adistance that is longer than distance W2 away from the other jointregion 46, 47. In each of joint regions 45, 46 and 47, the sum of theperimeters of joint parts 40 is made longer than the perimeter ofrectangle 65 of minimum area enclosing therein all of adjacent jointparts 40.

In the above-mentioned embodiment, attention is focused on therelationship between all of joint parts 40 of flat battery 10 andrectangle 63 of minimum area enclosing all these joint parts 40 so as toprevent breakage in joint parts 40 by satisfaction of La>Lb. Bycontrast, attention is focused on each joint region 45, 46, 47constituted by adjacent joint parts 40 for improvement in joint strengthin the present embodiment.

As shown in FIG. 11, the plurality of joint regions 45, 46 and 47 areprovided on flat battery 10. Each of joint regions 45, 46 and 47 isdefined as the region in which the plurality of joint parts 40 arearranged adjacent to each other. These joint regions 45, 46 and 47 arelocated at distance W5 from each other. Distance W5 is longer thandistance W2 between adjacent joint parts 40. Holding portion 50 isformed between joint parts 40 and between joint regions 45, 46 and 47.

In one joint region 45, joint parts 40 are formed so as to satisfy thecondition of La (the sum of the perimeters of adjacent joint parts40)>Lc (the perimeter of rectangle 65 of minimum area enclosing thereinadjacent joint parts 40). This allows improvement in the tensilestrength in joint region 45 as compared to the case forming a continuousjoint in the region corresponding to joint region 45. As in the case ofjoint region 45, joint parts 40 are formed so as to satisfy thecondition of La>Lc in each of the other joint regions 46 and 47. Thisalso allows improvement in the tensile strength in each of joint regions46 and 47. The tensile strength of separator 22 can be improvedthroughout flat battery 10 by improving the tensile strength in eachjoint region 45, 46, 47.

As described above, it is possible according to the present anotherembodiment to refill the stacked electrode assembly with electrolyticsolution 51 from holding part 50 and prevent breakage in joint parts 40,whereby the secondary battery can secure battery performance duringlong-term use.

Although one joint region is constituted by four adjacent joint parts inthe present another embodiment, the number of joint parts in one jointregion is not particularly limited and can be adjusted as appropriate.The number of joint regions in one flat battery is not also particularlylimited and can be adjusted as appropriate although three joint regionsare provided in one flat battery in the present another embodiment.

Modifications

The present invention is not limited to the above embodiments. Variouschanges and modifications are possible within the scope of the presentinvention. Although both of positive and negative tabs 41 and 42 areprovided on one side of exterior member 30 in flat battery 20 in theabove embodiment, the present invention is applicable to the secondarybattery in which positive and negative tabs are provided on differentsides. It is alternatively feasible to provide holding part 50 and tabson the same side.

Examples

Examples of flat battery 10 with a plurality of joint parts 40 will benext described below. It is herein noted that the secondary batteryaccording to the present invention is not limited to the followingexamples.

The operation conditions of Examples and Comparative Example areindicated in TABLE 1.

TABLE 1 Dis- Number Fused Fused Width Length tance of joint Perim- jointW1 W3 W2 fused area eter strength (mm) (mm) (mm) joints (mm²) (mm) (N)Exam- 2 2 1 6 24 48 210 ple 1 Exam- 1 1 1 12 12 48 212 ple 2 Compa- 12 20 1 24 28 128 rative Exam- ple

In each of Examples 1 and 2, flat battery 10 was provided to satisfy thecondition: the sum of the perimeters of the fused joints (joint parts40) (La)>the perimeter of rectangle 63 of minimum area enclosing all offused joints 40. The fused joints of Example 1 and the fused joints ofExample 2 are respectively schematically shown in FIGS. 12(A) and 12(B).

Examples 1 and 2 were different from each other in terms of the size offused joints 40, the distance between fused joints 40 and the number offused joints 40. The total area of fused joints 40 was made larger inExample 1 than in Example 2. In each of Examples 1 and 2, holding part50 was formed between the plurality of fused joints 40.

In Comparative Example, a conventional flat battery with continuousfused joint 140 was provided (the number of fused joints was 1). In FIG.12(C), fused joint 140 of Comparative Example is shown. The total areaof fused joint 140 was the same as that of Example 2.

Flat batteries 10 of Examples 1 and 2 had a higher level of fused jointstrength than that of the conventional flat battery of ComparativeExample as is seen from TABLE 1. As is seen from comparison of Example 1and Comparative Example, flat battery 10 of Example 1 had a higher fusedjoint strength than that of Comparative Example even though there was nodifference in the total area of the fused joints. It has thus been shownby the above results that flat battery 10 according to the presentinvention can ensure improved strength against tensile force applied tofused joints 40 as compared to the conventional flat battery.

The invention claimed is:
 1. A secondary battery, comprising: a stackedelectrode assembly having a positive electrode, a negative electrode anda separator; an electrolytic solution, the separator having permeabilityto the electrolytic solution; exterior members accommodatingtherebetween the stacked electrode assembly together with theelectrolytic solution; and electrode terminals connected to the stackedelectrode assembly and led to the outside from the exterior members soas to take an electric current out of the stacked electrode assembly,wherein the exterior members have respective outer peripheral portionswith peripheral edges thereof directly sealed to each other; and whereinthe secondary battery further comprises: a plurality of joint partsformed between inner areas of the outer peripheral portions of theexterior members and aligned in a first direction parallel to therespective peripheral edges on one peripheral side of the exteriormembers, with first clearance regions respectively disposed between thejoint parts and a second clearance region disposed between therespective joint parts and the respective peripheral edges of theexterior members, such that the inner areas of the outer peripheralportions of the exterior members are joined to each other by the jointparts, and such that an outer peripheral portion of the separator isjoined to the inner areas of the outer peripheral portions of theexterior members by the joint parts, the first clearance regions beingaligned along the first direction, the second clearance region extendingalong the first direction in outer areas of the outer peripheralportions of the exterior members and being beyond the first clearanceregions in a second direction perpendicular to the first direction; andan electrolyte holding part formed in the first and second clearanceregions so as to hold therein the electrolytic solution.
 2. A secondarybattery, comprising: a stacked electrode assembly having a positiveelectrode, a negative electrode and a separator; an electrolyticsolution; and exterior members accommodating therebetween the stackedelectrode assembly together with the electrolytic solution and havingrespective outer peripheral portions with peripheral edges thereofdirectly sealed to each other, wherein the secondary battery furthercomprises: a plurality of joint parts formed between the outerperipheral portions of the exterior members and aligned in a firstdirection parallel to the respective peripheral edges on one peripheralside of the exterior members, with first clearance regions respectivelydisposed between the joint parts and a second clearance region disposedbetween the respective joint parts and the respective peripheral edgesof the exterior members, such that an outer peripheral portion of theseparator is joined to the outer peripheral portions of the exteriormembers by the joint parts, the first clearance regions being alignedalong the first direction, the second clearance region extending alongthe first direction in outer areas of the outer peripheral portions ofthe exterior members and being beyond the first clearance regions in asecond direction perpendicular to the first direction; and anelectrolyte holding part formed in the first and second clearanceregions so as to hold therein the electrolytic solution, wherein each ofthe outer peripheral portions of the exterior members includes aplurality of supporting parts coupled to a supporting member of thebattery and an extension part located between the supporting parts andextending outwardly of the battery; and wherein the outer peripheralportion of the separator includes a tongue part extending to theextension parts of the exterior members and on which the joint parts areformed.
 3. The secondary battery of claim 1, wherein a sum La ofperimeters of the plurality of joint parts is larger than a perimeter Lbof a rectangle of minimum area enclosing the plurality of joint parts.4. The secondary battery of claim 2, wherein a sum La of perimeters ofthe plurality of joint parts is larger than a perimeter Lb of arectangle of minimum area enclosing the plurality of joint parts.